This invention relates in a totally general manner to the manufacture of ceramic tiles, and more particularly concerns a method for loading powder materials into the relative forming moulds.
The invention also relates to the means for implementing said method, and the materials obtained thereby.
The ceramic tile manufacturing sector is known to constantly seek new and original ornamental motifs, and in particular decorations reproducing the appearance of natural stone, such as marble, which is known to present veining and elongate striations of various shapes and colours.
Decorative motifs reproducing said appearance typical of marble can be obtained by the modern ceramic technology involved in the manufacture of fine porcellainized sandstone, which is well known to the expert of this sector, and will therefore not be described in detail.
It is sufficient to state that such decorative motifs can concern either the entire bulk, i.e. the entire thickness of the tile, or just the layer located at the exposed face of said tile.
In particular, in the second case double loading is effected, the first loading using a base material of not particular value intended to form the tile support, whereas the second uses a finishing material, i.e. possessing properties such as to provide the desired characteristics of the exposed face of the tile.
Said second material can consist of at least two at least partly mixed powders having different characteristics, typically different colours.
The invention relates to both said loading methods.
For simplicity, express reference will be made hereinafter to tiles decorated throughout their bulk, it being however understood that that stated is also valid for tiles decorated through only a part of their bulk.
Such bulk-decorated tiles are known to be formed by moulds comprising at least one forming cavity which is filled by a suitable loading carriage provided with a loading compartment for retaining the powders, the loading compartment being usually provided with a grid.
The carriage is driven with horizontal reciprocating rectilinear movement between a retracted position in which it disposes the grid in correspondence with a powder supply station, and an advanced position in which it disposes the grid above said at least one forming cavity, where the powders fall by gravity.
In certain cases the powder mass consisting of at least two at least partly mixed materials having different characteristics, typically different colours, is directly loaded into the grid, whereas in other cases said two materials are contained in respective hoppers located above the grid.
In all cases the grid presents a capacity greater than that of the forming cavity, in order to obtain complete filling of said forming cavity, and hence the desired tile thickness.
Moreover the lower generators of the grid are normally positioned in line with the upper face of the die plate, which defines the upper edge of the forming cavity, in front of the grid there usually being provided a scraper which during the carriage retraction movement smoothes the material deposited in the forming cavity. In some cases the grid can be slightly spaced from the die plate.
Said carriage retraction movement causes excess material still present within the grid to slip onto the surface layer of the material present in the forming cavity, with the result that the original powder distribution is altered.
In particular said masses mix together to generate a surface layer or sheet of virtually uniform colour.
The resultant aesthetic effect is obviously unacceptable, to expose the tile decoration it then being necessary to carry out a grinding operation aimed at removing said surface layer of uniform colour in order to expose the true distribution of the underlying multi-colour powders.
This involves fairly considerable costs, due in particular to the necessary equipment, and problems related to the containing and disposal of the fine powders produced by such machining.
In addition it is not possible to produce tiles having irregular surfaces, for example raised or projecting portions reproducing the splits in natural stone, as said grinding destroys such irregularities.
An object of the invention is to provide a method able to overcome said problems, in particular able to eliminate said surface defects due to said slippage during the filling of the mould forming cavity, in order not to require subsequent finishing operations on the tile once fired.
Another object is to provide a method by which tiles can be obtained having their exposed face not only multi-coloured but also irregular, for example provided with projections recalling the splitting of natural stone.
Another object is to provide means for implementing said method within the context of a simple, rational, reliable, long-lasting and low-cost construction.
Said objects are attained by virtue of the characteristics indicated in the claims.